1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a laser printer, which transfers a toner image formed on an image bearing member to a recording material using an electrophotography technique. More specifically, the present invention relates to an image forming apparatus having a transfer belt that transfers an image and conveys a recording material.
2. Description of the Related Art
In an electrophotographic apparatus in which a transfer belt stretched over a plurality of rollers carries and conveys a recording material, the recording material on the transfer belt is electrostatically attracted to the transfer belt after having passed through a transfer nip.
However, if the recording material has a low rigidity, the curvature of a separation roller, serving as a separation belt-stretching member, around which the transfer belt is stretched and the rigidity of the recording material itself are not enough to separate the recording material from the transfer belt. In such a case, the recording material is adhered to the transfer belt at a position of the separation roller, resulting in a separation error. Japanese Patent Laid-Open No. 9-015987 discloses a configuration in which uniform protrusions are formed on the surface of the separation roller, around which the transfer belt is stretched, to form unevenness (projecting portions and recessed portions) at the separation position of the transfer belt to separate the recording material. Although unevenness can be formed at the separation position of the transfer belt in this configuration, the transfer belt is constantly and locally subjected to a large tension. As a result, the transfer belt is locally worn away, causing variations in the resistance and making the transfer characteristics unstable.
Japanese Patent Laid-Open No. 5-119636 discloses a method in which a sheet that carries a recording material is deformed to separate the recording material while preventing wearing thereof due to deformation. The method disclosed therein uses rollers, serving as a push-up member, which can be moved between a position where they can push up the transfer sheet from the inside and a position where they do not push up the transfer sheet. In the method disclosed therein, the rollers push up the transfer sheet to separate the recording material, and the rollers do not push up the transfer sheet when the recording material is not separated.
If this configuration is applied to the transfer belt, a push-up member that can locally push up the transfer belt using a roller during a separation process is disposed downstream of a transfer member that transfers a toner image to a recording material on the transfer belt, in the conveying direction of the recording material. If the recording material has a low rigidity, as in the case of using a thin sheet, the transfer belt is locally pushed up and conveys the recording material. Thus, unevenness is formed in the recording material, increasing the rigidity of the recording material during the separation process.
The separated recording material is conveyed along the extension of the belt surface. Accordingly, in one configuration, a guide member for guiding the separated recording material is disposed so as to be flush with the belt surface that is not pushed up. However, when the push-up member pushes up the belt to form unevenness, from a position where the unevenness is formed to the position of the separation belt-stretching member, the recording material is conveyed along the belt surface from the recessed portion to the separation belt-stretching member due to the gravity. After separated, the recording material is conveyed along the extension of the belt surface from the recessed portion to the separation belt-stretching member. The extension of the belt surface from the recessed portion to the separation belt-stretching member is shifted toward the separation belt-stretching member, compared with the belt surface that is not pushed up. In this case, if the guide member for guiding the recording material separated from the belt is disposed so as to be flush with the belt surface that is not pushed up, a conveyance error of the recording material to be conveyed to the guide member may occur when the recording material is separated by a pushing-up operation.